This invention relates to video signal processing devices. More particularly, this invention relates to video signal processing devices for converting video signals from one format to a second format.
Video standard conversion is a known technique used to convert video signals from one standard to another. Typically, video programming material prepared according to a first standard (e.g. NTSC encoded signals) must be converted to another different standard (e.g. PAL or SECAM) in order to render the programming material compatible with the signal processing equipment for the converted standard. For example, if a program has been recorded using the NTSC format, the video signals recorded on the tape are incompatible with a VCR designed to process PAL or SECAM formatted signals. Consequently, the NTSC version must first be converted to the PAL or SECAM standard and then recorded or displayed. Similarly, video programming information which is broadcast using signals encoded according to a PAL format cannot be received and displayed with SECAM or NTSC equipment. Such signals must first be converted from the PAL standard to the standard used by the receiving equipment.
The basic requirement for standards conversion is to change the frame repetition rate and the number of lines per frame to different values required by the standard to which the signals are being converted. NTSC formatted signals, for example, have a frame rate of 30 frames per second and 525 lines per frame. SECAM formatted signals have a frame repetition rate of 25 frames per second and 625 lines per frame. Thus, to convert from NTSC to SECAM formatted signals, the frame repetition rate must be changed from 30 frames per second to 25 frames per second, and the number of lines per frame must be changed from 525 lines to 625 lines. To reduce the frame rate, some frames must be deleted on a predictable basis. Correspondingly, to increase the total number of lines, 100 lines must be added to each frame. When discarding the frames, averaging techniques are typically used in order to avoid the jerkiness which would result if a single frame were simply discarded abruptly. Similarly, when adding lines, the new lines are usually created by averaging adjacent lines in order to avoid distortions of the frame image and to provide a full span of information in the vertical direction.
When converting signals from PAL or SECAM format to NTSC format, the frame rate must be increased by 5 frames per second and the line rate must be decreased by 100 lines per frame. The frame rate increase is normally handled by averaging adjacent frames on a periodic basis to create interpolated frames; while the line decrease is usually done by averaging some adjacent lines to create interpolated lines. While line averaging does not substantially impair the image quality, both interframe averaging and interpolation tend to reduce the resolution of the individual frames affected and introduce artifacts which can be quite noticeable to the viewer. In many instances, the interframe averaging effects accumulate, which causes further degradation in the picture resolution.
Typically, known standards conversion techniques have been used in the past to change the format of video signals from one standard to another to provide compatibility between foreign signals and domestic equipment. Recently, however, a new problem has arisen in connection with a special effects video processor unit which has recently become available. This special effects processor unit, which is sold under the trademark "Video Toaster" by NewTek has achieved acceptance and success in the marketplace as a relatively low cost special television production system capable of a wide variety of standard and custom special effects. In particular, at a relatively low cost, the Video Toaster device can produce special effects heretofore only possible with units selling for nearly ten times the price of the Video Toaster unit. Special effects such as dissolves, wipes, fades and keying can all be done using the Video Toaster unit in combination with a relatively low cost personal computer.
A significant disadvantage of the Video Toaster unit lies in the fact that the unit is only compatible with NTSC formatted video signals. Thus, applications of the Video Toaster unit have been limited to video signals formatted according to the NTSC standard. Efforts to combine a conventional standards converter with the Video Toaster processing unit have not been successful due to the introduction into the original video signals of unacceptable judder and resolution degradation. This is due to the manner in which known standards converters normally operate by interframe averaging and interpolation. Since the purpose of the Video Toaster unit is to manipulate video signals from at least two sources having different programming content, lack of judder and high resolution are important characteristics of the resulting processed signals. Ideally, no artifacts should be introduced into the video signals during special effects processing, and the resolution should be equal to that of the input video signals. This problem is exacerbated by the requirement of a second standards converter, which is needed to convert the processed signals output from the Video Toaster unit back to the original format, and which adds another layer of image degradation.